The invention relates to a cooling element, in particular for use in walls of furnaces that are subjected to high levels of thermal stress, consisting of cast copper or a low-alloyed copper alloy, with coolant channels which comprise tubes cast in the copper or the copper alloy and are arranged inside the said cooling element.
The invention also relates to a method for producing a cooling element provided inside with coolant channels formed from tubes, in particular for use in walls of furnaces that are subjected to high levels of thermal stress, with the steps of    a) fabricating the tube, including all desired curves, branches and similar flow structures,    b) casting molten copper or copper alloy around the tubes within a casting mold, with preferably simultaneous cooling of the inner walls of the tubes,    c) cooling the copper melt.
Such cooling elements are usually arranged between the casing and the lining of a furnace, often also for use behind the refractory lining, for which purpose the cooling elements are connected to the cooling system of the furnace, for example a pyrometallurgical smelting furnace. The surfaces of these cooling elements may, as described for example in EP 0 816 515 A1, be provided on the side facing the interior of the furnace with additional webs or grooves or honeycomb-shaped depressions, in order in this way to permit a better bond with the refractory lining of the furnace or to ensure good adhesion of the slag or metal that is produced by the process in the furnace and solidifies on account of intensive cooling by the cooling elements, as a protection for the cooling element against chemical attack and against erosion. The cooling elements are usually used in the form of cooling plates in the region of the furnace walls or the roof or the hearth region of cylindrical or oval kilns. Such cooling elements are similarly used for pig-iron blast furnaces, in electric arc furnaces, direct reduction reactors and fusion gasifiers. Further areas for use of the cooling elements are burner blocks, tuyeres, casting cavities, electrode clamps, tapping-hole blocks, hearth anodes or dies for anode molds.
The aim in principle with the cooling elements is to achieve a high degree of heat dissipation, whereby both the lifetime of the cooling elements can be improved and peak thermal loads of the process in the furnace, in particular in dynamic operation, which lead to destruction of the cooling element, can be avoided.
In the case of cooling elements with cast-around tubes as coolant channels, the aim is not only for good flow guidance, as free from loss as possible, but also for good heat transfer from the cast metal of the cooling element to the cooling fluid flowing in the tubes. The already cited EP 0 816 515 A1 proposes for this purpose achieving an improved bond between the tube and the casting compound by making part of the thick-walled copper tubes begin to melt when the liquid copper is cast around them, which however entails considerable process-engineering difficulties, since the tube and the melt have the same melting point because they are made of essentially the same material. In the case of relatively cold casting, there is the risk of the tube not fusing adequately with the poured-in metal. This has the consequence of a very great heat transfer resistance between the tube and the cast-around metal. If, conversely, the casting temperature is increased, localized dissolution and melting-through of the tubes, or at least indentation of the cross section of the tubes, is scarcely avoidable, even if thick-walled tubes are used. A composite cast body produced like this is unusable in a furnace.
When copper melts are used, metallurgical dependencies also play an important part. Copper melts tend to absorb gases. In the casting process, disturbing effects are caused in particular by hydrogen and oxygen. The duration of the melting time, and possibly the overheating temperature, likewise play a part and may vary from melting process to melting process. Hydrogen and oxygen are in equilibrium with each other, for which reason high oxygen contents are accompanied by low hydrogen contents and vice versa. Because the solubility of hydrogen in solid copper is much less than in liquid copper, it can be followed from this that the solubility for hydrogen significantly decreases as the temperature falls. At the transition from the liquid phase into the solid phase of the copper melt, an extremely great reduction in the solubility for hydrogen takes effect, generally being referred to as a sudden drop in solubility when the temperature falls below the liquidus temperature, amounting to about 3.5 ml of oxygen per 100 g of copper melt.
The temperature and the pressure also play a major part in determining the absorbency of a melt for gases. The casting of a hydrogen-containing copper melt in the presence of oxygen on the tube surface in the form of copper oxide is problematical, since the oxygen in the atmosphere permeates the melt during the casting on account of the extremely rapid heating up of the tube. On account of the sudden drop in solubility at the transition of the melt from its liquid state into the solid state, the hydrogen set free reacts with the copper oxide in that the latter is reduced and the water vapor produced causes a gas porosity of the casting. From a process-engineering aspect, this can be counteracted by vacuum degassing, which however involves additional effort. Alternatively, a shifting of the water-oxygen equilibrium in the direction of oxygen can be achieved by deliberate oxygen charging, and with it removal of the hydrogen. Following the oxidizing treatment of the melt, the oxygen content must be deliberately reduced by performing a deoxidizing treatment of the melt in the ladle. On account of this albeit laborious two-stage metallurgical treatment of the copper melt, a reaction with the oxygen of the copper oxide of the cast-around copper tubes can no longer lead to an undesired formation of water vapor and consequently gas bubbles within the melt.
As already described, the contact of a highly heated copper melt with a copper tube arranged in the casting mold causes the copper tube to be mechanically weakened. The tube has the tendency to be indented at those places that are subjected to the load of a higher column of metal. To overcome this difficulty, it is disclosed in DE-C 726 599 to pass gases or liquids under an increased counterpressure through the tubes during the casting, this counterpressure corresponding approximately to the deforming resistance of the tube at the softening temperature. However, even if this method is applied, oxidation of the tube on its outer surfaces cannot be avoided during the casting operation.
Various alternatives for the choice of material of the cast tubes are described in U.S. Pat. No. 6,280,681. Apart from describing the possibilities, but also the limits, of the use of tubes made of steel, high-grade steel and copper, also described is a type of cooling element for which tubes made of a material known commercially as “Monel” are used. This material has a copper content of 31% and a nickel content of 63%. It is also described in this publication that, to achieve a good bond, not only copper tubes can be used but also tubes made of Cu—Ni alloys, such as for example UNS C 71500 with a copper content of 70% and a nickel content of 30%. On account of their higher melting point, these tubes have the advantage of a higher thermal load-bearing capacity during casting and can often also be produced without at the same time passing cooling water through the tubes during and after the casting. With such tubes, the risk of the copper melt breaking through into the interior of the tube can be significantly reduced. To preserve a free tube diameter, the tubes are filled with sand before the casting, in order in this way to maintain the tube cross section and avoid collapsing of the tube. Unfortunately, the said tubes made of Cu—Ni and Ni—Cu alloys have a much poorer thermal conductivity than copper tubes, as a result of which significantly less heat can be dissipated when they are later operated as a cooling element, and thermal overloading can occur in particular in the regions of the furnace wall. Furthermore, alloys of nickel and copper are much more rigid, for which reason they cannot be shaped and bent as well. In critical regions, such as for example tight 180° bends, significantly more welds have to be provided on account of the use of pre-bent bends, thereby increasing the risk of later leakages, quite apart from the higher fabrication costs.
Furthermore, there is the already described risk of increased gas porosities on account of water vapor formation, which likewise makes the quality of the casting deteriorate, restricts the heat removal and thereby reduces the heat conduction, since the gas bubbles in the casting act like insulators. The differing coefficient of thermal expansion of the metals involved is also disadvantageous. Compressive and tensile stresses occur on the tube embedded in the casting mold, which can, depending on the shaping of the tube, lead to a locally poor bond between the tube and the cast-around copper, and consequently in turn to a deterioration in the heat conduction.
The prior art also includes a cooling element such as that described in GB 1 386 645. In the case of this cooling element, the tube to be surrounded by casting is not in the casting mold from the outset, but instead the copper melt for producing the copper block is initially introduced into the casting mold, and then the prefabricated tube is immersed in this melt, the inner walls of the tube at the same time being cooled. For the case in which the tube and the melt consist of different metals, the provision of an additional layer on the outer side of the tube is proposed, this additional layer consisting of a further, third metal, which can for example be electrodeposited on the tube. Which metals are suitable for such purposes remains open.